Reverse osmosis or nanofiltration process for cleaning water

ABSTRACT

A system for cleaning feed water of variable quality, the system comprising an inlet for selectively delivering feed water to one or other of at least two feed chambers, each feed chamber having a delivery pipe for delivering feed water to a reverse osmosis or nanofiltration; a pump to deliver the feed water from one of the chambers through its associated delivery pipe to the reverse osmosis or nanofiltration to create a concentrated feed stream and a product water stream; return pipes for selectively returning the concentrated feed stream to one or other of the at least two feed chambers; a product water outlet for removal of the product water; and means for switching the delivery of the concentrated feed stream between the selectable return pipes upon detection of a predetermined reduction in efficiency within one or another of the feed chambers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/777,207, filed on May 17, 2018, which is a 371 of PCT/IB2016/054172,filed Jul. 13, 2016, which claims the benefit of GB patent application1512979.4, filed Jul. 23, 2015 each of which is hereby incorporated byreference in its respective entirety as if fully set forth herein.

This invention relates to an improved method and system for the cleaningor desalination of feed water by reverse osmosis (RO) or nanofiltration(NF) in an open circuit.

TECHNICAL FIELD

Desalination by reverse osmosis (RO) occurs when salt water solution iscompressed against semi-permeable membranes at a pressure higher thanits osmotic pressure. An example of this process is the “Plug-FlowDesalination” method which involves passing of pressurized feed flowthrough pressure vessels having semi-permeable membranes. The feed thenseparates into a non-pressurized flow of desalted permeate and apressurized flow of brine effluent. Generally, the brine effluent is awaste product.

Nanofiltration (NF) is also a semi-permeable membrane filtration-basedmethod that uses nanometer sized cylindrical through-pores.Nanofiltration can be used to treat all kinds of water including ground,surface, and wastewater. Nanofiltration membranes have the ability toremove a significant fraction of dissolved salts.

The recovery rate achieved in the aforementioned processes depends uponthe quality of the feed water and applied pressure. Generally, feedwater is fed to the system for providing a waste brine stream and aproduct water stream.

It is an aim of the present invention is to provide an improved methodfor the cleaning or desalination of feed water by reverse osmosis ornanofiltration in an open circuit that can process feed water ofdifferent qualities and deal with different recovery rates.

It is a further aim of the present invention to provide an improvedsystem for the cleaning or desalination of feed water by reverse osmosisor nanofiltration in an open circuit that can process feed water ofdifferent qualities and deal with different recovery rates.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the present invention provides a methodof cleaning feed water (FW) of variable quality, the method comprising:

-   -   (a) delivering feed water (FW) to one of at least two feed        chambers;    -   (b) pumping feed water from one of the feed chambers through a        reverse osmosis (RO) or nanofiltration (NF) membrane to create a        concentrated feed stream and a product water stream (PW);    -   (c) reducing the pressure of the concentrated feed stream;    -   (d) returning the concentrated feed stream to the original feed        chamber for delivery back through the reverse osmosis or        nanofiltration;    -   (e) switching the return delivery of the concentrated feed        stream to the at least one other feed chamber upon detecting a        predetermined reduction in the efficiency of the RO or NF        process within the original feed chamber;    -   (f) removing the concentrated feed (CW) from the original feed        chamber and delivering fresh feed water to this chamber during        continuous circulation of the feed water from the at least one        other feed chamber through the reverse osmosis or nanofiltration        back to the at least one other feed chamber; and    -   (g) passing the feed stream through a desaturation unit prior        to, or after, its passage through the reverse osmosis or        nanofiltration

Preferably, the method comprises switching delivery of the concentratedfeed stream from the at least one other feed chamber to the originalfeed chamber upon detecting a predetermined reduction in the efficiencyof the RO or NF process within the at least one other feed chamber,removing the concentrated feed (CW) from the at least one other feedchamber and delivering fresh feed water (FW) to this chamber.

This enables cleaning of the feed chamber to take place during removalof the concentrated feed stream therefrom, while feed water continues tobe fed to the reverse osmosis or nanofiltration chamber from the otherfeed chamber.

Any appropriate desaturation unit may be used to remove contaminants,such as dissolved salts and sparingly soluble salts from the feed streamprior to, or after, its passage through the reverse osmosis ornanofiltration. Examples include fluidised bed reactors, softeners, ionexchangers and/or an absorber.

The reduction in efficiency of the RO or NF process may be detected in avariety of ways. Preferably, detection of a predetermined maximum saltconcentration in the chamber causes switching of the return delivery tothe at least one other feed chamber. More preferably, the predeterminedmaximum salt concentration corresponds to the maximum osmotic pressureat which the reverse osmosis or nanofiltration can operate.

Preferably, the step of reducing the pressure of the concentrated feedstream prior to its return delivery to one or other of the feed chambersreduces the pressure of the concentrated feed stream is reduced tosubstantially atmospheric pressure. This may be achieved by an open loopwherein the feed stream is passed back to a chamber that is open toatmosphere. Alternatively or additionally, a pressure exchanger may beused to reduce the pressure of the concentrated feed stream. Passing thefeed stream through a desaturation unit may occur prior to, or after,this pressure reduction.

The method may also include pre-treating the feed water prior to itsdelivery to the reverse osmosis or nanofiltration. For example, thepre-treatment may comprise filtering the feed water prior to itsdelivery to the reverse osmosis or nanofiltration. Furthermore, thefiltered feed water may be pumped at high pressure through the membrane.

According to a second aspect of the present invention, there is provideda system for cleaning feed water of variable quality, the systemcomprising:

-   -   (a) an inlet for selectively delivering feed water (FW) to one        or other of at least two feed chambers, each feed chamber having        a delivery pipe for delivering feed water to a reverse osmosis        or nanofiltration;    -   (b) a pump to deliver the feed water from one of the chambers        through its associated delivery pipe to the reverse osmosis (RO)        or nanofiltration (NF) membrane to create a concentrated feed        stream and a product water stream (PW);    -   (c) return pipes for selectively returning the concentrated feed        stream to one or other of the at least two feed chambers;    -   (d) a product water outlet for removal of the product water        (PW);    -   (e) means for switching the delivery of the concentrated feed        stream between the selectable return pipes upon detection of a        predetermined reduction in efficiency of the RO or NF process,        such as detection of a maximum salt concentration, within one or        other of the feed chambers; and    -   (f) a desaturation unit provided in at least one feed stream        between the feed chamber and the reverse osmosis or        nanofiltration or in at least one return pipe between the        reverse osmosis or nanofiltration and the feed chamber.

The switching means is preferably adapted to enable the delivery of feedwater from a first chamber through a first delivery pipe to the reverseosmosis membrane to be recycled through its return pipe to the firstchamber until the predetermined reduction in efficiency is detected inthat chamber whereupon the switching means enables feed water to bedelivered from a second chamber though a second delivery pipe to thereverse osmosis or nanofiltration to be recycled through its return pipeto the second chamber until a predetermined reduction in efficiency isdetected in the second chamber.

Preferably, the switching means also activates removal of concentratedfeed water from the feed chamber upon detection of the predeterminedreduction in efficiency, such as upon detection of a maximum saltconcentration within that chamber, and the delivery of fresh feed waterto the reverse osmosis or nanofiltration from the other feed chamber.

Additionally, the switching means may activate the delivery of freshfeed water to the chamber following the removal of the concentrated feedstream from that chamber.

The desaturation unit is provided in either the feed streams between thefeed chamber and the reverse osmosis or nanofiltration, or in the returnpipes between the reverse osmosis or nanofiltration brine and the feedchamber. Any suitable type of desaturation unit may be provided.

Preferably, the system according to the second aspect of the presentinvention is an open loop system wherein the pressure of theconcentrated feed stream in the return pipes is reduced by passing thefeed stream to chambers that are open to atmosphere. Additionally oralternatively, a pressure exchanger may be provided within the system.Preferably, the pressure of the concentrated feed stream in the returnpipes is reduced to substantially atmospheric pressure. The desaturationunit may be provided between the pressure exchanger and the feedchamber.

The system may include a pre-treatment unit, such as a filter unit, forpre-treating the feed water prior to its delivery to the reverse osmosisor nanofiltration.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention shall now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a water cleaning system according toone embodiment of the present invention;

FIG. 2 is a schematic diagram of a water cleaning system according toanother embodiment of the present invention; and

FIG. 3 is a flow diagram illustrating the steps of the method accordingto a preferred embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides an improved reverse osmosis and/ornanofiltration method and system for enabling variable quality feedwater to be used with different recovery rates.

Referring to FIG. 1 of the accompanying drawings, one embodiment of asystem for cleaning feed water of variable quality is illustrated. Theembodiment illustrates the invention in relation to a reverse osmosisprocess and system but a nanofiltration membrane may be used as analternative to the reverse osmosis membrane. Feed water or salt water(FW) is introduced into a first feed chamber 2 from which it is directedthrough a delivery pipe 2 i to a desaturation unit 20 (for example, inthe form of a softener, ion exchanger or an absorber) followed by apre-treatment unit 50, such as filter unit. A high pressure pump 6 thenpressurizes the pre-treated feed water prior to its passage through areverse osmosis membrane 8 from which product water PW is produced,together with a concentrated brine stream CW. Normally, the brine streamwould then be discarded.

In the present invention, the concentrated brine stream CW is deliveredback to the first feed chamber via a pressure exchanger 40 in which itspressure is reduced back to substantially atmospheric pressure. Thesystem is also an open loop wherein the chambers are open to atmosphere.The concentrated brine stream is mixed with additional feed water in thefirst chamber and then recycled back through the system to provide moreproduct water PW and concentrated brine CW for recycling back to thechamber 2.

The system is provided with means for monitoring the efficiency of thereverse osmosis process. In this respect, it is to be appreciated thatrepeated recycling of the brine stream will reduce the efficiency of theprocess over time as the concentration of the feed water increases. Toaddress this issue, the system is provided with a second feed chamber 4.When the concentration of the feed water in the first chamber 2 reachesa predetermined level, the delivery pipe 2 i is shut and feed water isintroduced into the system from a second chamber 4 via delivery pipe 4i. This feed water is then passed through the desaturation unit 20 andpre-treatment unit 50, pumped through the reverse osmosis membrane 8 toprovide concentrated brine and product water PW. The concentrated brineis recycled back to the second chamber 4 via the pressure exchanger 40and a return pipe 4R for recycling through the system with further feedwater.

While feed water is being introduced from the second chamber, the highlyconcentrated brine water CW in the first chamber is removed via outletpipe 2 o. The chamber is cleaned and fresh feed water is introduced intothe chamber 2.

The system continues to monitor the efficiency of the reverse osmosisprocess. Over time, the feed water from the second chamber reaches apredetermined concentration, preferably being around the maximum osmoticpressure at which the reverse osmosis membrane can operate, at whichpoint the inlet 4 i of the second chamber is closed and feed water isagain delivered through the system from the first chamber 2 back to thefirst chamber via the pressure exchanger 40 and return pipe 2R. Theconcentrated brine in the second chamber is removed via outlet 4 o andfresh water is delivered into the second chamber 4.

In this manner, the system is able to cope with feed water of differentquality and work with different recovery rates.

It is to be appreciated that more than two feed chambers may be providedworking consecutively to allow recycling and cleaning of the feed water.Multiple chambers working in consecutive groups may also be provided inthe system.

The desaturation unit (20) may only come into play when the recycledfeed water reaches a predetermined salt concentration. Alternatively,the unit may be operational at all times. The unit may be providedelsewhere in the system, for example after the pressure exchanger 40 inthe return line, as shown in FIG. 2 of the accompanying drawings.

The system is preferably provided with appropriate electronic controlmeans for automatically switching between delivery of feed water fromthe respective chambers upon detection of predetermined reduction in theefficiency of the overall process, for example, corresponding to aparticular concentration being detected within each feed chamber.

FIG. 3 of the accompanying drawings illustrates the basic steps of amethod according to the present invention, again described in relationto a reverse osmosis process but the invention is also applicable tonanofiltration. Initially feed water is delivered to a first chamberfrom which it is pumped through a RO membrane to provide a clean productwater PW and a concentrated feed water. The pressure of the concentratedfeed water exiting the RO membrane is reduced to atmospheric pressure tothat is can be recycled back to the open first chamber for forming partof feed water (see “A” in FIG. 3 ). This cycle is repeated until theconcentration of the feed water in this chamber reaches a predeterminedlevel, at which point the water is removed, the chamber cleaned andfresh water is introduced into the first chamber (see “B”).

During removal of the water of the first chamber, feed water isintroduced into the system from a second chamber. Again the feed wateris pumped through the RO membrane and then recycled back to the secondchamber via a pressure exchanger for forming part of the feed water (see“C” in FIG. 3 ). This cycle is repeated until the concentration of thefeed water in this second chamber reaches a predetermined level. Thewater is then removed, the chamber cleaned and fresh water is introducedinto the second chamber (see “D”). During removal of the water, feedwater is again introduced from the first chamber and recycled asillustrated by steps A in FIG. 3 until the concentration reaches apredetermined level, at which point feed water is introduced from thesecond chamber and recycled as illustrated in steps C.

Ideally, the method further includes an additional step of removing thesalts from the feed water, either before or after its passage throughthe RO membrane. This may be achieved using any suitable desaturationunit, such as one containing a softener, ion exchanger or an absorber.

We claim:
 1. A system for cleaning feed water of variable quality, thesystem comprising: an inlet for selectively delivering feed water to oneor other of at least two feed chambers, each of the at least two feedchambers having an associated delivery pipe for delivering feed water toa reverse osmosis or nanofiltration membrane; a pump to deliver the feedwater from one or other of the at least two feed chambers through itsassociated delivery pipe to the reverse osmosis or nanofiltrationmembrane to create a concentrated feed stream and a product waterstream; return pipes for selectively returning the concentrated feedstream to one or other of the at least two feed chambers; a productwater outlet for removal of the product water; means for switching thedelivery of the concentrated feed stream between the return pipes upondetection of a predetermined reduction in the efficiency of the RO or NFprocess within one or other of the at least two feed chambers; at leastone desaturation unit provided in at least one return pipe between thereverse osmosis or nanofiltration brine and one or other of the at leasttwo feed chambers; and at least one pressure exchanger for reducingpressure of the concentrated feed stream prior to feeding theconcentrated feed stream through the at least one desaturation unit. 2.The system of claim 1, wherein the switching means is adapted to enablethe delivery of feed water from a first chamber through a first deliverypipe to the reverse osmosis or nanofiltration to be recycled through itsreturn pipe to the first chamber until the predetermined reduction inefficiency of the RO or NF process is detected in that chamber whereuponthe switching means enables feed water to be delivered from a secondchamber though a second delivery pipe to the reverse osmosis ornanofiltration to be recycled through its return pipe to the secondchamber until the predetermined reduction in efficiency is detected inthe second chamber.
 3. The system of claim 1, wherein the switchingmeans also activates removal of concentrated feed water from a firstfeed chamber of the at least two feed chambers upon detection of thepredetermined reduction in efficiency of the RO or NF process within thefirst feed chamber and the delivery of fresh feed water to the reverseosmosis or nanofiltration from a second feed chamber of the at least twofeed chambers.
 4. The system of claim 3, wherein the switching meansactivates delivery of fresh feed water to the chamber following theremoval of the concentrated feed stream from that chamber.
 5. The systemof claim 1, wherein the desaturation unit is provided in both feedstreams between the feed chamber and the reverse osmosis ornanofiltration and/or in a return pipe between the reverse osmosis ornanofiltration and a feed chamber.
 6. The system of claim 5, wherein thedesaturation unit is selected from the group consisting of a fluidizedbed reactor, a softener, an ion exchanger, an absorber, and combinationsthereof.
 7. The system of claim 1, further comprising an open loopsystem wherein the pressure of the concentrated feed stream in thereturn pipes is reduced by passing the feed stream to chambers that areopen to atmosphere.
 8. The system of claim 1, wherein the reduction inpressure of the concentrated feed stream is to substantially atmosphericpressure.
 9. The system of claim 1, further comprising a pre-treatmentunit for pre-treating the feed water prior to its delivery to thereverse osmosis or nanofiltration.
 10. The system of claim 9, whereinthe pre-treatment unit comprises a filter unit.
 11. The system of claim1, wherein a monitor performs the detection of the predeterminedreduction in the efficiency of the RO or NF process.
 12. The system ofclaim 11, wherein the monitor monitors when the feed water from the oneor other of the at least two feed chambers reaches a maximum osmoticpressure at which the RO or NF membrane can operate.
 13. A system forcleaning feed water, the system comprising: a first feed chamber and asecond feed chamber, each of the feed chambers for receiving feed waterand each of the feed chambers comprising a delivery pipe for deliveringthe feed water; a pump to deliver the feed water from the first feedchamber through the first delivery pipe through a reverse osmosis (RO)or nanofiltration (NF) membrane, thereby generating a concentrated feedstream and a product water stream; a first return pipe and a secondreturn pipe, each of the return pipes for selectively returning theconcentrated feed stream to the feed chambers; a product water outletfor removal of the product water stream; a pressure exchanger forreducing pressure of the concentrated feed stream prior to feeding theconcentrated feed stream through at least one desaturation unit; aswitching mechanism for switching delivery of the concentrated feedstream between the first return pipe and the second return pipe upondetection of a predetermined reduction in efficiency of the RO or NFmembrane; and the at least one desaturation unit provided in at leastone of the return pipes between the RO or NF membrane and at least oneof the feed chambers, wherein the feed water passes through the at leastone desaturation unit prior to, or after, its passage through the RO orNF membrane.
 14. The system of claim 1, wherein the predeterminedreduction occurs when the feed water from the one or other of the atleast two feed chambers reaches a predetermined concentration.
 15. Thesystem of claim 14, wherein the predetermined concentration occurs whena maximum osmotic pressure at which the RO or NF membrane can operate isreached.
 16. The system of claim 13, wherein the pump pressurizes thefeed water before delivery through the RO or NF membrane, and whereinthe pressure exchanger reduces the pressure of the concentrated feedstream to atmospheric pressure.
 17. The system of claim 13, wherein thepressure exchanger is disposed only in the concentrated feed stream. 18.The system of claim 13, further comprising a monitoring unit formonitoring efficiency of the RO or NF membrane, and wherein themonitoring unit performs the detection of the predetermined reduction inefficiency of the RO or NF membrane.
 19. The system of claim 13, whereinthe switching mechanism is electronically controlled.